In automated coating systems, for example of the type having one or more controlled material application spray guns (such as powder spray guns) positioned adjacent to a conveyor which carries parts to be coated past the guns, the guns are controlled (turned on and off and in some cases moved relative to the parts) to apply an optimal spray pattern to parts. Whenever there is a gap between parts on the conveyor, the guns should be turned off to stop spraying in order to minimize waste of material such as powder coating material. The turning on and off of the guns is referred to as "gun triggering". Automatic gun triggering gives the ability to optimize material usage. Automatic gun triggering uses a set of adjustable parameters which allow each gun of an array to automatically and efficiently apply a coating material to the parts. The automatic gun triggering parameters specify when and for how long each gun will spray a specific part.
In prior art systems of this type, the setting of optimal gun triggering parameters is difficult. An initial activation of the gun or guns has conventionally been accomplished by use of a photoeye detector which "sees" a part as it passes by on the conveyor. The first spray gun of an array is located at a fixed point from the photoeye. Many variables must be accounted for in achieving optimum spray coating, such as part size and shape, rate of conveyance, and spacing between parts. In a straight, uncalibrated automatic spraying set-up, the guns are turned on precisely at the front edge of a part, and turned off precisely at the back edge. This type of gun control may not completely coat the front and back edges of the part. Therefore, the triggering of the gun may have to be controlled to turn on prior to arrival of the part, and to continue spraying after the part has passed for complete coating of the part. This is called an "extended wrap" pattern which starts spraying prior to arrival of the leading edge of a part, and continues spraying after the part has passed. The gun on and gun off signals must be separately determined for extended wrap patterns. For example, if the guns were set to turn on before arrival of a part, this could cause the guns to turn off prior to arrival of the trailing edge of the part at the gun. A "delay off" control is used for keeping the guns spraying until the trailing edge of the part has passed. In other gun triggering set-ups, it is desired to start spraying after the leading edge of the part has passed the gun. This is referred to as "restricted wrap" pattern. In this case, a "delay-on" control is required to prevent the gun from spraying at the leading edge of the part. In this case also, the total spray duration must be adjusted to account for the delay-on control.
Prior art systems therefore have required setting of three different gun triggering control parameters: Delay-off (for extended wrap), Delay-on (for restricted wrap), and a parameter known as the "Pickoff". The Pickoff is the parameter which determines when the gun will start spraying relative to the position of a part approaching the gun. The distance "D" is the distance from the photoeye to the gun. If the Pickoff is set to equal the distance "D" then the gun will start spraying exactly when the part reaches the gun (i.e., when a leading edge of the part is aligned with the gun). The gun will start spraying at the Pickoff point, and spray for the length of the part, i.e., for the amount of time it takes the part to pass the gun G. The length of the part is automatically detected as the part is carried through the photoeye on the conveyor and this information is input to the controller. If the Pickoff is set at less than the fixed distance D, and no adjustments are made, the gun would start spraying before the forward edge of the part passes the gun, spray for the length of the part and stop spraying before the part has passed the gun. If the Pickoff is set at greater than the distance "D", the gun will start spraying after the part reaches the gun, spray for the length of the part, and stop spraying after the rear edge of the part has passed the gun.
In the prior art, as schematically shown in FIG. 1, in order to spray a distance "X" before the part reaches the gun, and to continue spraying after the part has passed the gun for a distance "Y", the following setup is required:
Pickoff=D-X PA1 Delay-on=0 (since this is an extended wrap pattern and Delay-on is only used for restricted wrap patterns) PA1 Delay-off=X+Y PA1 Pickoff=D PA1 Front-Edge-Spray=X PA1 Back-Edge-Spray=Y
The Pickoff, the part position at which the gun will start spraying, (D-X) is less than the distance "D". The gun will start spraying the distance "X" before the leading edge le of the part reaches the gun. Spray continues for the distance X plus the length "P" of the part plus the distance Y after the trailing edge of the part passes the gun. Thus, the Delay-off is set at X+Y meaning the controller holds the guns open for the distance of X+Y conveyor movement in addition to the distance P which is the length of the part. To achieve this spray pattern, a D-X Pickoff value and X+Y Delay-off value are input to the controller.
One difficulty of this prior art gun triggering procedure lies in the fact that both the Pickoff and the Delay-off are functions of the same variable X. If there is a change in the distance "X", the distance in front of the part that the gun begins spraying, both the Pickoff and the Delay-off parameters must be adjusted and input again into the controller. The Pickoff and the Delay-off parameters are not independent.
A numerical example of a prior art spray pattern is also shown in FIG. 1. In a case where the system is to apply a coating to part P such as a flat panel which is 20 inches long, in order to coat the leading and trailing edges of the panel it is desired to start spraying 6 inches (of conveyor travel) before the leading edge le of the part P arrives at gun G, and to continue spraying for 4 inches (of conveyor travel) after the trailing edge te of the part passes gun G. This "extended wrap" spray pattern covers a total distance of 30 inches (6"+20"+4"). In other words, 30 inches of the conveyor on which the part is mounted passes the gun G during the spray period.
If the gun is set to start spraying 6" before the leading edge of the part reaches the gun G, the pickoff must be adjusted by subtracting 6" from the 100" distance D between the photoeye PE and gun G. Therefore, an "adjusted pickoff" of 96" (D-X) is input to the controller. To continue spraying for 4" after the part passes the gun, the "Delay-off" of 10" (6"+4") is also input to the controller The length of the part P=20" is read by the photoeye as the part passes the photoeye and also input to the controller. When the leading edge of the part P is 6" from the gun, the gun will spray for 30" of conveyor travel (6"+20"+4"). The gun will stop spraying 4" after the trailing edge te of the part P passes the gun G.
If a change in the setup is made to spray, for example, 9" before the leading edge, and still 4" after the trailing edge, the length of spray before the leading edge must be changed, from 6" to 9". The Pickoff is set to start spraying 9" before the leading edge le of the part reaches the gun G. Therefore, 100"-9"=91" is now input to the controller as the Pickoff. The Delay-off value must also be adjusted to 9"+3"=12" and input into the controller. The part length of 20" is unchanged. With these new parameters, the gun will start spraying 9" before the leading edge of the part arrives at the gun and will spray continuously for 9"+20"+4"=33" of conveyor travel. Thus, to generate this new triggering pattern, the Pickoff must be adjusted and reinput as well as the Delay-off. This makes this triggering system difficult and time consuming to configure and re-configure especially for different types of parts combined on a single conveyor.
Another problem associated with automated spray systems of the prior art is the inability of the operator to watch the spraying operation to confirm proper coating coverage. This is because a stationary control panel is typically located adjacent to a spray booth wherein the spray guns are located. Ideally, an operator is able to observe parts as they pass through the booth past the guns to assess the pattern and extent of coating coverage. However, since the operator must remain at the control panel, in some installations it is difficult to see from the panel location the parts as they are coated. This makes it more difficult to properly set the spraying parameters.